The present invention relates to a method of detecting the arrangement of a plurality of beam spots in a multi-beam imaging apparatus.
A multi-beam imaging apparatus typically divides a beam emitted by a light source into a plurality of beams directed to a surface to be scanned. Specifically, the plurality of divided beams are converged to form beam spots on the surface, respectively. By scanning the beam spots with respect to the surface in a direction which is different from the direction where the beam spots are arranged, and by modulating respective beams, a two dimensional image is formed on the surface.
In this type of multi-beam imaging apparatus, in order to make the intervals between scanning lines formed by the scanning beam spots, it is important that the plurality of beam spots are arranged evenly, in particular in the auxiliary scanning direction, on the surface to be scanned. If the beam spots are unevenly arranged, the scanning lines are not formed at an even pitch, and the quality of the formed image may be deteriorated.
An optical system of the multi-beam imaging apparatus is therefore designed such that the beam spots are evenly arranged on the surface. However, due to aberrations of the optical system and/or manufacturing errors of the optical system, there may be some unevenness in the interval between the beam spots. Therefore, it is necessary to detect the arrangements of the beam spots after the optical system is assembled, and adjust the intervals therebetween.
Conventionally, the intervals are detected based on the intervals between the scanned lines as printed. Alternatively, a photo-electric conversion element such as a CCD (Charge Coupled Device) is placed at a certain position so that the plurality of the beams spots are formed thereon, and the positions of the beams are directly detected.
However, in the former method, the result of the adjustment cannot be known immediately. That is, until the scanning lines are printed, whether the adjustment is appropriate or not is unknown. If the adjustment is insufficient, the adjustment is performed again, and then the scanning lines should be printed again to check whether the adjustment is appropriate. Thus, it takes time and it is troublesome if the former method is taken. According to the latter method, since each beam spot is detected as an intensity distribution, it is difficult to determine a position of the beam spot or a center of the beam spot accurately. The center of gravity may be assumed as the center of the beam spot. However, the intensity distribution may include individual disturbance, and in such a case, if all the beam spots are treated in a similar manner, the arrangement of the beam spots may not be adjusted accurately.
It is therefore an object of the invention to provide an improved method of detecting an arrangement of a plurality of beam spots instantly and accurately.
For the above object, according to the invention, there is provided a method of detecting a positional relationship of a plurality of beam spots for a multi-beam imaging apparatus, the imaging apparatus including a light source emitting a beam, a beam splitter that receives the beam emitted by the light source and divides the beam into a plurality of beams, a deflecting system that deflects the plurality of beams to scan, and an imaging optical system that converges the plurality of scanning beams on a surface to form the plurality of beam spots. According to the method, a phase filter is provided between the light source and the beam splitter. The filter is configured to divide a cross section of the beam directed from the light source to the beam splitter into a plurality of areas. Light fluxes passed through adjoining two of the plurality of areas of the phase filter have an optical path difference of half a wavelength, so that at least a dark line is formed in each beam spot due to the phase difference of the adjoining two areas. A positional relationship of dark lines respectively formed on the plurality of beam spots are detected to determine a positional relationship between the plurality of beam spots.
According to this method, only by inserting a phase filter, the arrangement of a plurality of beam spots can be detected instantaneously.
The dark lines thus formed and bright areas on either sides of each dark line provides a high contrast portion, which enables an accurate detection of the beam position. Further, since the phase filter is inserted on an upstream side of the beam splitter, the dark lines appear similarly in all the beam spots. Accordingly, relative positions of the beam spots can be detected accurately.
In one particular case, the phase filter divides a cross section of the beam into four areas in a circumferential direction with respect to the center thereof. Each adjoining two areas provides an optical path difference of half a wavelength to the light passed therethrough. A dark line is formed on the beam spot at a portion corresponding to a boundary of each adjoining two areas.
In this case, the dark lines intersect at one point. Therefore, the position of each beam can be detected easily.
Optionally, the four areas are substantially evenly arranged. In this case, a cross-shaped dark lines are formed on each beam spot.
Alternatively, the phase filter may include a central area and at least one surrounding area surrounding the central area. Light fluxes passed through the central area and the at least one surrounding area have an optical path difference of half a wavelength. Dark lines are formed one the beam spot at portions corresponding to a boundary of the central area and the at least one surrounding area.
Optionally, the central area may be a circular area, and the surrounding area is an annular area.
In this case, within a circular bright area (i.e., a beam spot), a concentric dark circle is observed. Thus, the position of each beam spot can be detected easily.
According to another aspect of the invention, there is provided a phase filter used for detecting a positional relationship of a plurality of beam spots in a multi-beam imaging apparatus. The imaging apparatus includes a light source emitting a beam, a beam splitter that receives the beam emitted by the light source and divides the beam into a plurality of beams, a deflecting system that deflects the plurality of beams to scan, and an imaging optical system that converges the plurality of scanning beams on a surface to form the plurality of beam spots. The phase filter includes a plurality of filtering areas to which the light beam is incident, and a cross section of the light beam is divided into a plurality of divided areas by the plurality of filtering areas. Light fluxes passed through adjoining two of the plurality of filtering areas of the phase filter have an optical path difference of half a wavelength.
If the filter is inserted in an optical path between the light source and the beam splitter, detection of an arrangement of the beam spots on the surface to be scanned can be performed easily.
Optionally, the filter is configured to divide a cross section of the beam into four areas in a circumferential direction with respect to the center thereof. Each adjoining two areas provide an optical path difference of half a wavelength to the light passed therethrough.
The four areas may be divided by two lines perpendicular to each other.
In another case, the filter has a central area and at least one surrounding area that surrounds the central area. Light fluxes passed through the central area and the at least one surrounding area have an optical path difference of half a wavelength.
The central area and the at least one surrounding area may be divided by a circle.